Application of Liesegang Ring Formation on a Gelatin Film to the Determination of Sulfate Concentration in Individual Rain Droplets

Abstract

The determination of SO₄^2- concentration in individual rain droplets can be achieved using a gelatin film containing 5 mM barium chloride and 5 mM barium nitrate. SO₄^2- in droplets reacts with barium chloride in the film to form the characteristic Liesegang rings. The areal ratio of the ring to the droplet print (ARRD) on the film is a good indicator for determining the SO₄^2- concentrations in an individual droplet. Interference by NO₃^- in determining SO₄^2- concentration can be avoided by adding barium nitrate to the gelatin. Measurements of SO₄^2- in rainwater samples by this method agreed well with those from ion chromatography. The individual analysis of rain droplets by this method indicated that the concentrations of SO₄^2- were largely different with a 5-order magnitude, even among individual rain droplets collected simultaneously, and were higher in smaller rain droplets.